When connected to a system, a power conversion device outputs harmonic currents in addition to a fundamental frequency current. The harmonic currents may cause overheating and breakdown of a power-factor-improving capacitor, burning of a transformer, beat of an electric motor, or malfunctioning of a breaker, wherein the power-factor-improving capacitor, the transformer, the electric motor, and the breaker are connected to the system. Accordingly, a regulation is provided for harmonic outflow when a power conversion device is connected to a system. In order to suppress harmonic current output, a power conversion device is connected to an LC filter.
However, it is possible that resonance between the LC filter and the capacitor, transformer or load in the system occurs to increase a specific order harmonic current output. For such a situation, there is a technique of adding a resistance to a filter capacitor in series, to suppress the harmonic current. Suppression of harmonic current may be implemented by applying damping control to the power conversion device, and applying a filter to an inverter output current sensing signal, to extract harmonics and adjust the phase, and adding same to an output voltage command value to cancel the harmonics (see patent document 1).
Moreover, in paragraphs 0005 and 0006 of the specification of patent document 2, it is described that an inverter control is performed with a feedback of a current of an output section of an LC filter in order to maintain a distortion current suppression effect even when the value of the inductance component of the system changes during resonance suppression control. Patent document 2 discloses a method of sensing the filter current and compensating for harmonics. In this control, a phase offset is added to inverse dq transformation that is used to extract a harmonic, for taking into account a transfer characteristic in phase from current command value to filter current.
Moreover, there is a method of measuring a transfer function from harmonic suppression current command value to output current of AC filter, for suppressing harmonic currents outputted from a power conversion device.